The filtration properties of nonwoven polymeric fibrous webs can be improved by transforming the web into an electret, i.e., a dielectric material exhibiting a quasi-permanent electrical charge. Electrets are effective in enhancing particle capture in aerosol filters. Electrets are useful in a variety of devices including, e.g., air filters, face masks, and respirators, and as electrostatic elements in electro-acoustic devices such as microphones, headphones, and electrostatic recorders.
Electrets are currently produced by a variety of methods including direct current (xe2x80x9cDCxe2x80x9d) corona charging (see, e.g., U.S. Pat. No. Re. 30,782 (van Turnhout)), and hydrocharging (see, e.g., U.S. Pat. No. 5,496,507 (Angadjivand et al.)), and can be improved by incorporating fluorochemicals into the melt used to produce the fibers of some electrets (see, e.g., U.S. Pat. No. 5,025,052 (Crater et al.)).
Many of the particles and contaminants with which electret filters come into contact interfere with the filtering capabilities of the webs. Liquid aerosols, for example, particularly oily aerosols, tend to cause electret filters to lose their electret enhanced filtering efficiency (see, e.g., U.S. Pat. No. 5,411,576 (Jones et al.)).
Numerous methods have been developed to compensate for loss of filtering efficiency. One method includes increasing the amount of the nonwoven polymeric web in the electret filter by adding layers of web or increasing the thickness of the electret filter. The additional web, however, increases the breathing resistance of the electret filter, adds weight and bulk to the electret filter, and increases the cost of the electret filter. Another method for improving an electret filter""s resistance to oily aerosols includes forming the electret filter from resins that include melt processable fluorochemical additives such as fluorochemical oxazolidinones, fluorochemical piperazines, and perfluorinated alkanes. (See, e.g., U.S. Pat. No. 5,025,052 (Crater et al.)). The fluorochemicals should be melt processable, i.e., suffer substantially no degradation under the melt processing conditions used to form the microfibers that are used in the fibrous webs of some electrets. (See, e.g., WO 97/07272 (Minnesota Mining and Manufacturing)).
In one aspect, the invention features an electret that includes a surface modified polymeric article having surface fluorination produced by fluorinating a polymeric article. In one embodiment, the article includes at least about 45 atomic % fluorine as detected by ESCA. In another embodiment, the article includes a CF3:CF2 ratio of at least about 0.25 as determined according to the Method for Determining CF3:CF2. In other embodiments, the article includes a CF3:CF2 ratio of at least about 0.45 as determined according to the Method for Determining CF3:CF2.
In one embodiment, the article has a Quality Factor of at least about 0.25/mmH2O, (preferably at least about 0.5/mmH2O, more preferably at least about 1/mmH2O).
In some embodiments, the article includes a nonwoven polymeric fibrous web. Examples of suitable fibers for the nonwoven polymeric fibrous web include polycarbonate, polyolefin, polyester, halogenated polyvinyl, polystyrene, and combinations thereof. Particularly useful fibers include polypropylene, poly-(4-methyl-1-pentene), and combinations thereof. In one embodiment, the article includes meltblown microfibers.
In another aspect, the invention features an electret that includes a polymeric article having at least about 45 atomic % fluorine as detected by ESCA, and a CF3:CF2 ratio of at least about 0.45 as determined according to the Method for Determining CF3:CF2. In another embodiment, the electret includes at least about 50 atomic % fluorine as detected by ESCA, and a CF3:CF2 ratio of at least about 0.25 as determined according to the Method for Determining CF3:CF2.
In other aspects, the invention features a respirator that includes the above-described electrets. In still other aspects, the invention features a filter that includes the above-described electrets.
In one aspect, the invention features a method of making an electret that includes: (a) fluorinating a polymeric article to produce an article having surface fluorination; and (b) charging the fluorinated article in a manner sufficient to produce an electret. In one embodiment, the method includes charging the fluorinated article by contacting the fluorinated article with water in a manner sufficient to produce an electret, and drying the article. The method is useful for making the above-described electrets. In another embodiment, the method includes charging the fluorinated article by impinging jets of water or a stream of water droplets onto the fluorinated article at a pressure and for a period sufficient to produce an electret, and drying the article.
In other embodiments, the method includes fluorinating a polymeric article in the presence of an electrical discharge (e.g., an alternating current corona discharge at atmospheric pressure) to produce a fluorinated article. In one embodiment, the method includes fluorinating the polymeric article in an atmosphere that includes fluorine containing species selected from the group consisting of elemental fluorine, fluorocarbons, hydrofluorocarbons, fluorinated sulfur, fluorinated nitrogen and combinations thereof. Examples of suitable fluorine containing species include C5F12, C2F6, CF4, hexafluoropropylene, SF6, NF3, and combinations thereof.
In other embodiments, the method includes fluorinating the polymeric article in an atmosphere that includes elemental fluorine.
In other embodiments, the method of making the electret includes: (A) fluorinating a nonwoven polymeric fibrous web (i) in an atmosphere that includes fluorine containing species and an inert gas, and (ii) in the presence of an electrical discharge to produce a web having surface fluorination; and (B) charging the fluorinated web in a manner sufficient to produce an electret.
In other aspects, the invention features a method of filtering that includes passing an aerosol through the above-described electrets to remove contaminants.
The fluorinated electrets of the invention exhibit a relatively high oily mist resistance relative to non-fluorinated electrets.
In reference to the invention, these terms having the meanings set forth below:
xe2x80x9celectretxe2x80x9d means a dielectric material exhibiting a quasi-permanent electrical charge. The term xe2x80x9cquasi-permanentxe2x80x9d means that the time constants characteristic for the decay of the charge are much longer than the time period over which the electret is used;
xe2x80x9csurface modifiedxe2x80x9d means that the chemical structure at the surface has been altered from its original state.
xe2x80x9csurface fluorinationxe2x80x9d means the presence of fluorine atoms on a surface (e.g., the surface of an article);
xe2x80x9cfluorine containing speciesxe2x80x9d means molecules and moieties containing fluorine atoms including, e.g., fluorine atoms, elemental fluorine, and fluorine containing radicals;
xe2x80x9cfluorinatingxe2x80x9d means placing fluorine atoms on the surface of an article by transferring fluorine containing species from a gaseous phase to the article by chemical reaction, sorption, condensation, or other suitable means;
xe2x80x9caerosolxe2x80x9d means a gas that contains suspended particles in solid or liquid form; and
xe2x80x9ccontaminantsxe2x80x9d means particles and/or other substances that generally may not be considered to be particles (e.g., organic vapors).